Sources of phase coherent short pulses of light have a wide range of applications, for example in sensing or metrology apparatus such as strain, pressure or temperature sensors, spectroscopy apparatus or distance measurement apparatus. Sources of short light pulses are also used in optical communications applications, for example applications using phase shift encoding with a modulation technique such as quadrature phase shift keying.
One of the applications of sources of phase coherent short pulses of light is in quantum communications systems. In a quantum communication system, information is sent between a transmitter and a receiver by encoded single quanta, such as single photons. Each photon carries one bit of information encoded upon a property of the photon, such as its polarization, phase or energy/time. The photon may even carry more than one bit of information, for example, by using properties such as angular momentum.
Quantum key distribution (QKD) is a technique which results in the sharing of cryptographic keys between two parties; a transmitter, often referred to as “Alice”, and a receiver, often referred to as “Bob”. The attraction of this technique is that it provides a test of whether any part of the key can be known to an unauthorised eavesdropper, often referred to as “Eve”. In many forms of quantum key distribution, Alice and Bob use two or more nonorthogonal bases in which to encode the bit values. The laws of quantum mechanics dictate that measurement of the photons by Eve without prior knowledge of the encoding basis of each causes an unavoidable change to the state of some of the photons. These changes to the states of the photons will cause errors in the bit values sent between Alice and Bob. By comparing a part of their common bit string, Alice and Bob can thus determine if Eve has gained information.
QKD systems which use phase-encoding can employ an asymmetric Mach-Zehnder interferometer (MZI) at the transmitter to encode the phase information. The MZI contains a beam splitter, which divides light pulses into two fibres. The fibres then recombine on a second beam splitter. The separate fibres are labelled the short arm and the long arm. A phase modulator can be installed on either the long arm or the short arm in order to encode the phase information in the pulses. The optical path length difference between the short arm and the long arm should match that of a second MZI which is used to decode the information at the receiver.